01 - In a heatproof bowl, vigorously whisk together the eggs and granulated sugar until fully blended.
02 - Mix in the lemon zest and freshly squeezed lemon juice until homogeneous.
03 - Place the bowl over a saucepan with gently simmering water, ensuring that the bowl’s bottom does not contact the water.
04 - Constantly whisk the mixture over the steam until it thickens to coat the back of a spoon, approximately 8 to 10 minutes.
05 - Remove from heat and promptly whisk in the cubed butter until completely melted and smooth.
06 - Pass the curd through a fine-mesh sieve into a clean bowl to ensure a silky texture free of zest or coagulated egg.
07 - Allow to cool at room temperature, then transfer to sterilized jars. Refrigerate and consume within two weeks.